This invention relates generally to papermaking fabrics. It relates more specifically to papermaking fabrics used in the forming section of a papermaking machine.
Papermaking machines transform an aqueous slurry of fibers into a continuous paper web which can be processed for a variety of end uses. Papermaking fabrics are employed throughout the papermaking process to transport the web of paper as a continuous sheet through the papermaking equipment.
The papermaking process starts in the forming section of a papermaking machine where the aqueous slurry is deposited onto a forming fabric having the desired characteristics for retaining the fibers while allowing the water to pass through. The wet paper web created by this process is then carried by a press fabric through the press section where additional water is removed by squeezing the paper web and fabric between two rolls. The paper web is then carried through the drying section on a dryer fabric to remove additional water through forced evaporation. The design of papermaking fabrics used on each section of a papermaking machine vary in accordance with function.
In the forming section of papermaking machines, the fibers are retained and collected on the upper surface of a forming fabric and formed into a paper sheet. The forming fabric must have a fine mesh weave on the paper contact side in order to avoid marking the paper and to support the fiber from the slurry. The fabric must also have good drainage characteristics for initial water removal to facilitate paper formation. However, as previously noted, the forming fabric also serves as a drive belt and is subjected to high tensile loads in the machine direction and compressive or buckling loads in the cross machine direction. Therefore, a single fine-mesh yarn system is generally not suitable for use as a forming fabric.
To combat prior art problems, multi-layer forming fabrics were developed with fine-mesh yarn on the paper forming surface to facilitate paper formation, and larger yarns on the machine contact side to provide strength and longevity. Example multi-layer forming fabrics include U.S. Pat. No. 4,709,732 which discloses a dual layer forming fabric for use in the paper making process and U.S. Pat. No. 4,606,585 which discloses a two-ply forming fabric with a two-shaft, twill or satin weave pattern. U.S. Pat. No. 5,025,839 discloses a two-ply forming fabric with zig-zagging machine direction yarns which provides improved drainage. U.S. Pat. No. 5,857,498 also discloses a two-ply forming fabric which claims to include zig-zagging machine direction yarns. Both of the later fabrics include nonuniformly spaced paper side knuckles which may result in uneven spacing of the cross machine direction yarns and a configuration referred to as xe2x80x9ctwinningxe2x80x9d, both of which can result in uneven drainage and a nonuniform paper product. Additionally, each of these fabrics has a twill bottom surface which may result in xe2x80x9cguidingxe2x80x9d of the fabric on the papermaking machine.
The present invention relates to a double layer papermaking fabric preferably for use in the forming section of a papermaking machine. The fabric has a paper side cross machine direction layer having twice as many yarns as the machine side cross machine direction layer. A system of machine direction yarns is interwoven with both cross machine direction layers, with each machine direction yarn forming first and second knuckles on each side of the fabric. The paper side knuckles are uniformly spaced to reduce the likelihood of xe2x80x9ctwinningxe2x80x9d. Each machine side cross machine direction yarn is passed under by two machine direction knuckles spaced by at least one intermediate machine direction yarn, thereby producing a machine direction yarn zig zag effect on the machine side of the fabric to provide improved drainage.